Research is concerned with the role of mandibular proprioceptors (muscle spindle and temporomandibular joint afferents) and the precentral cortex in the control of mandibular position and isotonic movement. Monkeys are trained to maintain a specified jaw position, either with or without visual feedback, in the face of variations in the opening or closing loads placed on the mandible. Deficits in the ability of the animals to maintain position by proprioceptive cues will be determined before and after lesions of the muscle spindle afferent fibers, anesthetization of the temporomandibular joints, or after a combination of these two procedures. Activity of muscle spindle afferent fiber cell bodies, located in the mesencephalic nucleus of the fifth nerve, will be related to jaw position and movement under various loading conditions. The activity of cells in the face area of the precentral cortex will be similarly studied, and the responsiveness of these cortical cells to stimulation of jaw muscle and joint afferents will be tested.